ABSORPTION OF THE VARIOUS FOOD SUBSTANCES 



447 



it is probable that everything which is useful in the nutrition of 

 the body is taken up by the cells, while such substances as metallic 

 salts which are foreign to the organism, and are denied entrance 

 into the cells, may pass in small amount between them, their passage 

 being perhaps associated with more or less injury to the interstitial 

 substance. The vigilant selection exercised by the mucosa is well 

 illustrated by the facts that, although manganese and iron are 

 chemically so closely related, iron, which is necessary for the 

 formation of the blood-pigment, is absorbed in immensely greater 

 amount than manganese; and that chlorides, especially sodium 

 chloride, are readily taken up, sulphates with difficulty. Iron is 

 absorbed by the bloodvessels, but also to some extent by the lacteals. 

 From the blood it is carried to various organs, especially the spleen 

 and liver. There is reason to believe that the eosinophile leucocytes 

 take some share in its transportation. 



It was supposed by Bunge that only organic compounds of iron 

 could be absorbed, and that the undoubted benefit derived from 

 the administration of inorganic iron compounds, such as ferric 

 chloride, in chlorosis, was due not to their direct absorption, but 

 to their shielding the organic compounds from the attack of the 

 sulphuretted hydrogen in the intestine (p. 424). But this theory 

 has been shown to be inconsistent with the facts. For instance, 

 after the administration of salts of iron, the iron in the blood, 

 liver, spleen, and other organs increases, but there is no accumula- 

 tion of iron in the liver of an animal to which salts of manganese 

 have been given, although these are equally decomposed by sul- 

 phuretted hydrogen. 



Absorption of Proteins. 'The proteins of the food or their digested 

 products also pass directly into the blood-capillaries which feed 

 the portal system. For it has been shown that after ligature of 

 the thoracic duct protein substances are still absorbed from the 

 intestine, and the urea corresponding to their nitrogen appears in 

 the urine. The total nitrogen in the chyle flowing from a fistula 

 of the thoracic duct in a man was not found to be increased during 

 the digestion of protein food. The quantity of chyle escaping in a 

 given time was also unaffected, whereas during the digestion of fats 

 it was greatly augmented (Munk). 



Although a certain amount of egg-albumin, serum-albumin, alkali- 

 albumin, and other native or slightly altered protein substances 

 can be absorbed as such by the small, and even by the large, in- 

 testine, there is no evidence that, under ordinary conditions, this 

 mode of absorption is of any practical importance in nutrition, 

 although in another relation it may possess a certain interest (p. 32), 

 For when native proteins, with the possible exception of the 

 serum proteins from an animal of the same species, are introduced 

 ' parenterally,' so that they do not reach the tissues by way of 



